JPH09176792A - Heat treated steel parts and their production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily produce heat treated steel parts capable of being remarkably improved in fatigue strength while maintaining wear resistance. SOLUTION: An alloy steel, having a composition consisting of 0.10-0.40% C, 0.06-<0.15% Si, 0.30-1.00% Mn, 0.90-1.20% Cr, >0.30-0.50% Mo, and the balance Fe, is subjected to carburizing and quenching or to carbo-nitriding and quenching, by which the steel parts, in which the depth of surface abnormal layer is regulated to <= about 15μm, is easily formed. Subsequently, shot peening is applied to the steel parts, by which compressive residual stress can be formed in the steel surface to a greater extent than heretofore while preventing the occurrence of fatigue crack due to the notch effect resulting from surface roughness or the occurrence of deterioration in wear resistance caused by surface roughness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treated steel part and a method for manufacturing the same.

[0002]

2. Description of the Related Art For gears and the like of power transmission components, when high fatigue strength is required, shot pinning is performed after carburizing and quenching. This shot
This is intended to suppress the generation or propagation of fatigue cracks by forming a compressive residual stress on the steel surface by the thinning.

As a preferable example of the above shot peening, as disclosed in Japanese Patent Application Laid-Open No. Sho 60-218422,
Low carbon alloy steel SCM415, that is, C: 0.13-
0.18%, Si: 0.15 to 0.35%, Mn: 0.1%
60-0.85%, P ≦ 0.03, S ≦ 0.03, C
r: 0.90 to 1.20%, Mo: 0.15 to 0.30
%, The alloy steel consisting of the balance Fe is carburized and quenched, and the shot particle projection speed for the alloy steel is 35 to 50 m / sec.
c, Production method of performing shot pinning under the conditions of shot hardness HRC 45 to 50 is known. According to this, the compressive residual stress can be left while the surface roughness is suppressed to about 1 μm or less, which is allowable, and a constant fatigue strength can be obtained.

By the way, the higher the fatigue strength, the more
It is preferable for components requiring strength, and from this viewpoint, it is desirable that a fatigue strength equal to or higher than the fatigue strength obtained by the above-described manufacturing method be obtained. In such a case, it is necessary to further increase the compressive residual stress formed on the steel surface, and to increase the compressive residual stress, it is conceivable to increase shot hardness, shot particle projection speed, and the like as much as possible. .

[0005]

However, in order to improve the fatigue strength, shot-peening is performed on the above-mentioned alloy steel with a shot hardness, shot particle projection speed, etc. which are higher than those in the above-mentioned manufacturing method. , The surface roughness of the steel surface exceeds the permissible range, fatigue cracks occur due to the notch effect based on that roughness, or even the required compressive residual stress on the steel surface is not formed, fatigue The strength is rather decreased, and it is more difficult to increase the fatigue strength than in the case of the above manufacturing method.

[0006] Under such circumstances, the present inventor:
As a result of intensive studies on the above problems, they discovered that an incompletely quenched layer (abnormal surface layer) associated with grain boundary oxidation was involved, and found the following knowledge.

High-speed shot particle projection speed using a high-hardness shot (compared to conventional shot peening conditions)
In Shottopi - Doing training, Shottopi - depth of training before the average surface abnormal layer and Shottopi - relationship between the average surface roughness after training is obtained as a characteristic curve shown in FIG. 1 (the characteristic line f ₁ Shottopi - training conditions: shot hardness HRC54, shot particles blasting speed 90m / sec, shot size 0.6 mm, the shot time 120 sec, the characteristic line f ₂ Shottopi - training conditions:. shot hardness HR
C50, shot particle projection speed 60 m / sec, shot diameter 0.6 mm, shot time 120 sec. , Shottopi characteristic line f ₃ - training conditions: shot Hardness HRC5
8, shot particle projection speed 120 m / sec, shot diameter 0.6 mm, shot time 120 sec. In this case, since the surface roughness after shot peening and the surface roughness after shot peening required from the viewpoint of abrasion resistance are not more than about 1 μm because fatigue cracks do not occur due to the notch effect. To reduce the surface roughness to about 1 μm or less, the depth of the abnormal surface layer before shot peening should be about 15 μm.
It must be less than μm. When shot peening is performed under the same conditions as in FIG. 1, the relationship between the depth of the average surface abnormal layer before shot peening and the surface compressive residual stress after shot peening is obtained as a characteristic line shown in FIG. In this case, the shot
If the average surface anomalous layer depth before polishing is about 15 μm or less, the compressive residual stress formed on the steel surface after shot peening can be made particularly high.

The present inventors have completed the present invention based on the above findings. That is, a first object of the present invention is to provide a heat-treated steel part capable of dramatically improving fatigue strength while maintaining wear resistance. Also, the second
It is an object of the present invention to provide a method for manufacturing a heat-treated steel part that can easily manufacture the heat-treated steel part.

[0009]

In order to achieve the above first object, in the first invention, C: 0.10 to 0.4
0%, Si: 0.06 to less than 0.15%, Mn: 0.3
0 to 1.00%, Cr: 0.90 to 1.20%, Mo:
A steel part obtained by carburizing or carbonitriding quenching an alloy steel having a balance of more than 0.30% and not more than 0.50% and a balance of Fe, and a depth of a surface incompletely hardened layer is about 15 μm.
Shot-peening is applied to the following items, which is a heat-treated steel component.

With the above structure, a steel part having the above-mentioned components and a surface incompletely hardened layer (hereinafter referred to as "abnormal surface layer") having a depth of about 15 μm or less has a high hard shot and a high speed. Even if shot peening is performed at a shot particle projection speed or the like, the surface roughness does not exceed the allowable limit of about 1 μm due to the abnormal surface layer, and fatigue cracks may occur due to the notch effect based on the surface roughness. Alternatively, it is possible to prevent deterioration of wear resistance due to surface roughness. On the other hand, the kinetic energy of the shot particles is suppressed from being absorbed as the rough surface roughness of the abnormal surface layer, and the shot peening is performed at a high hard shot, a high shot particle projection speed, etc. The compression residual stress can be formed on the surface more than ever.

[0011] Further, an embodiment of the first invention (Claim 1) is as described in Claim 2.

Further, in order to achieve the second object, in the second invention, C: 0.10 to 0.40%, S
i: 0.06 to less than 0.15%, Mn: 0.30 to 1.
00%, Cr: 0.90 to 1.20%, Mo: 0.30
% And 0.50% or less and the balance Fe is carburized or carbonitrided and then shot-peened, which is a method for manufacturing a heat-treated steel part.

In the above-mentioned constitution, the amount of Mo which is effective for the hardenability and does not cause the grain boundary oxidation is set to the above range by increasing it from the conventional amount, while it significantly promotes the grain boundary oxidation and does not contribute to the hardenability. Since the amount of Si is set to be in the above range by reducing it from the conventional value, the depth of the abnormal surface layer of the alloy steel after carburizing or carbonitriding quenching is about 15 μm or less due to the action of both, and as described above, Even if shot pinning is performed on steel with high hardness shots, high-speed shot particle projection speed, etc., fatigue cracks occur due to the notch effect based on the surface roughness, or wear resistance decreases due to surface roughness. This means that compressive residual stress can be formed on the steel surface by such shot pinning.

Moreover, once the composition of the alloy steel is determined as described above, the depth of the abnormal surface layer of the alloy steel can be reduced to about 15 μm or less by the usual carburizing or carbonitriding quenching treatment.

The critical significance of each component of the above alloy steel is as follows.

C: C is a basic element necessary for imparting strength to steel, and is required to be 0.1% or more in order to secure the strength of the core portion (inside) by carburizing and quenching. However, if the C content exceeds 0.40%, the toughness decreases and becomes brittle, and the machinability also deteriorates. Therefore, C
Is set in the range of 0.10 to 0.40%.

Si: Si is an element having a remarkably strong tendency to promote the grain boundary oxidation which causes the formation of the abnormal surface layer,
If its content exceeds 0.15%, its adverse effect cannot be ignored. Moreover, Si is an element that does not contribute to hardenability. Therefore, it is preferable to reduce the Si content as much as possible. However, Si is used as a deoxidizing agent, or for lowering the melting point to reduce the melting energy, and therefore, the Si content in steelmaking is 0.
More than 06% is required. Therefore, the Si content is set in the range of 0.06 to less than 0.15%.

Mn: Mn is an element that promotes the grain boundary oxidation that causes the formation of the abnormal surface layer, and the smaller the content of Mn, the more desirable. However, if Si is less than 0.15%, M
If O is added 0.30% or more, Mn
There is no adverse effect even if it is 1.00% or less. If the Mn content is less than 0.30%, the hardenability of the core portion becomes insufficient.
For this reason, the content of Mn is set in the range of 0.30 to 1.00%.

Cr: Cr is also an element that promotes grain boundary oxidation that causes formation of an abnormal surface layer, and the smaller the content, the more desirable. However, Si less than 0.15% and Mo
Provided that 0.30% or more is added, Cr1.
Even if it is 20% or less, there is no adverse effect. Further, the Cr content is 0.
If it is less than 90%, the hardenability of the core part and the carburizing property are deteriorated. Therefore, the Cr content is 0.90 to 1.20.
It is set in the range of%.

Mo: Mo is an element that does not cause grain boundary oxidation and enhances hardenability, and contributes to the reduction of the abnormal surface layer. If the Mo content exceeds 0.50%, the effect tends to be saturated, and if the Mo content is less than 0.30%, the effect is low and the hardenability of the core part becomes insufficient. Therefore, the Mo content is set in the range of 0.30 to 0.50%. Mo also has the effect of toughening the metallographic structure itself, in addition to the effect of reducing the abnormal surface layer in the above range.

The embodiments of the second invention (Claim 3) are as described in Claims 4 and 5.

[0022]

Embodiments of the present invention will be described below.

FIG. 3 shows a process for producing a heat-treated steel part. In this manufacturing process, each step of cutting, forging, normalizing, machining, heat treatment, tempering, and shot peening of the material is performed in order, and in some cases, each of forging, normalizing, and tempering is performed. The steps are to be omitted. In the manufacturing method according to the present invention, among the above manufacturing steps, the heat treatment step and the shot pinning step are deeply related, and these steps will be described in detail, and the other steps will be described because they are known. Omit it.

First, in the heat treatment step, the alloy steel is carburized and quenched or carbonitrided and quenched.

The above alloy steel has a component of C: 0.30 to 0.30.
0.40%, Si: 0.06 to less than 0.15%, Mn:
0.30 to 1.00%, Cr: 0.90 to 1.20%,
Mo: More than 0.30% and 0.5% or less, with the balance being Fe. This component is determined from the viewpoint that the depth of the abnormal surface layer of the alloy steel after carburizing and quenching is about 15 μm or less, and the critical significance of each component is as described above.

The above-described carburizing and quenching and quenching and quenching can be performed under general conditions. For example, for carburizing and quenching, the carburizing temperature is 900 to 950 ° C., and the carburizing time is 0.5 to 5.0 hours. This example has a preferred carburized quench hardening depth (Hv> 550) of 0.2 to 1.3 m.
m. The range of this cure depth is
If it is less than 0.2 mm, the surface pressure resistance of the component is insufficient,
If the thickness exceeds 1.3 mm, the internal oxidation of the alloy element is remarkable, so that it is set in the above range.

By this ordinary carburizing or carbonitriding quenching, the depth of the abnormal surface layer formed on the alloy steel can be suppressed to about 15 μm or less, and no special treatment or treatment is required. The depth of the abnormal surface layer is about 15
μm or less. In particular, when carbonitriding and quenching is applied to the above alloy steel, the hardenability is improved by nitrogen,
This makes it possible to make the depth of the abnormal surface layer shallower than when carburizing and quenching.

Thus, the depth of the abnormal surface layer is about 15 μm.
It is as described above that the Si and Mo components of the alloy steel are greatly related to the suppression below, but this can be confirmed by the characteristic lines shown in FIGS. 4 and 5. That is, FIG. 4 shows the influence of the Si content on the depth of the abnormal surface layer. According to this, it can be understood that the lower the Si content, the shallower the depth of the abnormal surface layer. In this case, the respective components other than Si are steel types within the range considered to be substantially the same as shown in FIG.
Carburizing is performed at 930 ° C for 2 hours, and quenching is 85 after carburizing.
The tempering was performed from 0 ° C and the tempering was performed at 180 ° C for 1.5 hours.

FIG. 5 shows the effect of the Mo content on the depth of the abnormal surface layer. According to this, it can be understood that the greater the Mo content, the shallower the depth of the abnormal surface layer. In this case, each component other than Mo is a steel type within a range considered to be substantially the same as shown in FIG.
As in case i, carburizing is performed at 930 ° C. for 2 hours,
Quenching is performed at 850 ° C after carburizing, and tempering is 18
Performed at 0 ° C. for 1.5 hours.

FIG. 6 is a photomicrograph of 400 times showing the case where the depth of the abnormal surface layer is set to about 6 μm by the above method.
FIG. 7 shows that the depth of the surface abnormal layer is about 10 μm by the above method.
It is a microscope photograph figure of 400 times which shows the case where it was set to. 6 and 7, the lower side is the internal structure of the alloy steel,
The thin layer formed on the upper part of the alloy steel is the abnormal surface layer. On the other hand, FIG. 8 shows a conventional alloy steel (for example, SC
M415) shows the case where the depth of the abnormal surface layer is set to about 18 μm by performing normal carburizing and quenching.
FIG. 9 is a photomicrograph at × 100 magnification and FIG. 9 is a photomicrograph at × 400 magnification showing the case where the depth of the abnormal surface layer is set to about 25 μm by the same method as in FIG. 8 and 9, the lower side is the internal structure of the alloy steel, and the layer formed above the alloy steel is the abnormal surface layer.

In the shot peening step, the shot hardness HRC 50 to 58, the shot particle projection speed 60 to 120 m / sec, the shot diameter 0.1 to 1.0 mm, the shot peening time 10 to 3 are preferable.
Shot peening is performed under the condition of 00 seconds.

Regarding the shot hardness and shot particle projection speed, the larger the abnormal surface layer is about 15 μm or less, as described above, the more preferable it is from the viewpoint of forming compressive residual stress. However, the shot hardness is HRC
When it is lower than 50, the working force of shot-peening is insufficient to improve the fatigue strength, and HRC58
When it is higher than the above value, the shot-peening effect is saturated and the shot is easily cracked. When the shot particle projection speed is lower than 60 m / sec, the shot
The working force of the ning becomes insufficient to improve the fatigue strength, and when it is larger than 120 m / sec, the shot is liable to be broken and the economy is impaired. For this reason,
The shot hardness and shot particle projection speed are preferably set in the above ranges.

As for the shot diameter, when the shot diameter is 0.
When it is smaller than 1 mm, the distribution layer of the compressive residual stress becomes thin, and when the shot diameter is larger than 1.0 mm, the thickness of the distribution layer of the compressive residual stress becomes sufficient, but the compressive residual stress value of the surface layer becomes low. Therefore, as described above,
The shot diameter is set within a range of 0.1 to 1.0 mm.

For the shot peening time, 10
If less than a second, the effect of shot peening is insufficient,
After 300 seconds, the effect of shot peening saturates,
The economy will be impaired. Therefore, the shot peening time is set within the range of 10 to 300 seconds as described above.

As a result, a sufficient compressive residual stress can be formed on the surface of the steel part without roughening the surface roughness of the steel part, and the fatigue strength can be improved while maintaining the wear resistance. You can do it.

[0036]

EXAMPLE The effect based on the above embodiment can be supported by the experimental example shown in FIG. FIG. 12 shows the specific experimental conditions (chemical composition of alloy steel, etc.) and the results of each experimental example. The common experimental conditions are as follows.

Common experimental conditions: Experimental parts: Automotive transmission gear (module
2.50) Carburizing: After carburizing at 930 ° C for 2 hours,
Quenching was performed at 850 ° C., and thereafter, tempering was performed at 180 ° C. for 1.5 hours. Shot pinning conditions: shot diameter 0.6 mm, shot time 150 seconds

(Experimental Example 1) In Experimental Example 1, when Si is near the lower limit and Mo is at the upper limit within the set range of the present invention, if the alloy steel is carburized and quenched, the surface abnormality before shot-peening is observed. It shows that the depth of the layer is 6 μm, which is extremely shallow. In addition, in this Experimental Example 1, if shot-peening is performed on such an alloy steel within the set range of the present invention, the depth of the abnormal surface layer is about 15 μm or less, so that the surface roughness is within an allowable range. Within 1.0 μm,
50kg compressive residual stress required to improve fatigue strength
f / mm ² (in FIG. 12, the “−” sign means that the compressive residual stress remains in the alloy steel), which also indicates that the fatigue strength can be improved.

(Experimental Example 2) In Experimental Example 2, in the set range of the present invention, when Si and Mo are in the middle, by carburizing and quenching the alloy steel, the depth of the abnormal surface layer before shot-peening is obtained. Indicates that it is within the allowable range of 10 μm. In addition, this experimental example 2 is similar to the experimental example 1.
It also shows that the fatigue strength can be improved by shot pinning.

(Experimental Example 3) In Experimental Example 3, when Si is near the upper limit and Mo is near the lower limit within the scope of the present invention, if the alloy steel is carburized and quenched, the abnormal surface layer before shot peening is formed. It indicates that the depth is 14 μm, which is close to the allowable limit. Further, this Experimental Example 3 also shows that the fatigue strength can be improved by shot pinning, like the Experimental Examples 1 and 2.

(Experimental Examples 4, 5, 6) These Experimental Examples 4,
Nos. 5 and 6 have too much Si in the setting range of the present invention,
When the Mo content is too small, even if the alloy steel is carburized and quenched, the depth of the abnormal surface layer before shot peening exceeds the allowable limit of 15 μm. Further, in Experimental Examples 4 and 5, when the depth of the abnormal surface layer exceeds the permissible limit as described above, even if the shot hardness and the shot particle projection speed are within the set range of the present invention, the shot peak The surface roughness after hardening exceeds the allowable limit of 1.0 μm or less, and the compressive residual stress is also required to improve fatigue strength.
It does not exceed 0 kgf / mm ² , indicating that improvement in fatigue strength cannot be expected. The reason why the surface roughness after the shot peening exceeds the allowable range is that the soft surface abnormal layer is relatively thicker than the other surface layers, so that the shot hardness and the shot particle projection speed increase. It is considered that the reason why the surface is easily roughened and that the compressive residual stress after the shot peening is not more than necessary is that the kinetic energy of the shot particles is consumed as the surface abnormal layer is roughened.

In Experimental Example 6, when the depth of the abnormal surface layer exceeds the permissible limit and the shot peening conditions (shot hardness, projection speed) are out of the setting range of the present invention, the fatigue strength is improved. Is not expected.

(Experimental Example 7) In Experimental Example 7, when both Si and Mo are within the set range of the present invention, if the alloy steel is carburized and quenched, the depth of the abnormal surface layer before shot-peening is increased. Within the allowable range, the shot hardness HRC of 50 or less and the shot particle projection speed of 60 are obtained for the alloy steel.
It is shown that the fatigue strength cannot be improved when shot-peening is performed under the condition of less than m / sec.

Although the above embodiment describes carburizing and quenching, the present invention can also be applied to carbonitriding and quenching. In this case, the number of abnormal surface layers can be reduced, which is a preferable result.

[0045]

According to the first aspect of the present invention, it is possible to prevent the occurrence of fatigue cracks due to the notch effect based on the surface roughness and the deterioration of wear resistance due to the surface roughness, and increase the kinetic energy of shot particles. By doing so, it is possible to form a compressive residual force on the surface of the heat-treated steel component more than ever, and thus it is possible to provide a heat-treated steel component that can dramatically improve fatigue strength while maintaining wear resistance.

According to the second invention, not only the above heat-treated steel part can be obtained by the method, but once the components of the alloy steel are determined as described above, the conventional carburizing and carbonitriding quenching treatments are applied. Since the depth of the abnormal surface layer of the alloy steel can be set to about 15 μm or less, when the above heat-treated steel part is obtained, special treatment / treatment is performed to reduce the depth of the abnormal surface layer to about 15 μm or less. It is not necessary, and it is possible to avoid complication of the process for making the depth of the abnormal surface layer about 15 μm or less.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing the relationship between the depth of an average abnormal surface layer before shot peening and the average surface roughness after shot peening.

FIG. 2 is a characteristic diagram showing a relationship between a depth of an average surface abnormal layer before shot peening and a surface compressive residual stress after shot peening.

FIG. 3 is a diagram showing a manufacturing process of a heat-treated steel part.

FIG. 4 shows the depth and S of the abnormal surface layer before shot peening.
FIG. 4 is a characteristic diagram showing a relationship with an i content.

FIG. 5: Depth of surface abnormal layer before shot peening and M
FIG. 4 is a characteristic diagram showing a relationship with o content.

FIG. 6 is a 400 × photomicrograph showing a metallographic structure when the depth of the abnormal surface layer is about 6 μm.

FIG. 7 is a 400 × photomicrograph showing a metallographic structure when the depth of the abnormal surface layer is about 10 μm.

FIG. 8 is a photomicrograph of 400 times showing a metal structure in a case where the depth of the abnormal surface layer is about 18 μm.

FIG. 9 is a photomicrograph at 400 × showing the metallographic structure when the depth of the abnormal surface layer is about 25 μm.

FIG. 10 is a diagram showing the ratio of each component when obtaining the characteristic diagram of FIG. 4;

FIG. 11 is a diagram showing the ratio of each component when obtaining the characteristic diagram of FIG. 5;

FIG. 12 is a diagram showing an experimental example.

Claims (5)

[Claims] 1. C: 0.10-0.40%, Si: 0.
06-less than 0.15%, Mn: 0.30-1.00%,
Cr: 0.90 to 1.20%, Mo: 0.30% to 0.50% and 0.50% or less, a steel part obtained by carburizing or carbonitriding quenching an alloy steel consisting of the balance Fe, and having an incomplete surface A heat-treated steel part, characterized in that shot-peening is applied to a hardened layer having a depth of about 15 μm or less. 2. The steel part according to claim 1, wherein the hardening depth based on the carburizing quenching or carbonitriding quenching treatment is 0, 2 mm to 1.3 mm in a region where the hardening depth is larger than Vickers hardness 550. Heat treated steel parts characterized by being 3. C: 0.10-0.40%, Si: 0.
06-less than 0.15%, Mn: 0.30-1.00%,
Cr: 0.90 to 1.20%, Mo: more than 0.30% and 0.50% or less, the alloy steel consisting of the balance Fe is carburized or carbonitrided and quenched, and then shot pinning is performed. A method for manufacturing a heat-treated steel part characterized by the above. 4. The production of heat-treated steel parts according to claim 3, wherein the quenching temperature is 900 ° C. to 950 ° C., and the quenching time is 0.5 to 5.0 hours. Method. 5. The shot pinning according to claim 3, wherein the shot diameter is 0.1 mm to
1.0 mm, shot particle projection speed 60 to 120 m / s
A method for manufacturing a heat-treated steel part, which is performed under the condition of ec.